On 8 Jun 2020 at 17:48, xxxxxx@gmail.com wrote:

> Now, space is different.
>
> No hiding in space (your thermal gives you away and you can be found
> by visuals tracking transits). Not a lot of cover.

It's possible to reduce your IR emmission in several ways, but most
are *highly* directional. You may appear to be as cold as the 3K
background from some direction, but in others your signature will be
*greatly* enhanced.

The two methods that don't have this flaw have major flaws of their
own.

"Storing" the heat by dumping it into a "cold sink" is not possible
for an extended time (hours at most).

Well, you *could* try hiding in an iceball/comet, but that too has
drawbacks. Pretty much the same drawbacks as the second method.

The second method is a *huge* shell (balloon) around the ship. This
spread the same amount of energy over a *much* larger area, thius
reducing the signature to something plausible for a natural body.

Trouble with that is that if it's a known system, all the bodies that
size will have been recorded and a "new" body, especiall well inside
the system is going to be *very* suspicious.

Worse, you are stuck moving at a plausible speed for a natural body.
Which means it'll take you months, if not years to get where you want
to be.

> A big ship might be a big target, but at long distances, the size of
> targets may matter less (it's a fraction of a degree of angle to
> adjust) and for guided munitions, they aren't getting small enough to
> dodge those.

Dodging works differently in space, regardless of the weapon.

For guided munitions, it's a matter of max accel *and* total
available delta-V. If the ship has a higher accel, the odds of being
able to hit it are essentially nil unless the muntions are very
stealthy and lying in wait for the ship to come to them.

If the munitions have a higher accel, they have a *small* chance. It
all depends on agility *and* delta-V. Doesn't matter if you have a
higher accel if you run out of fuel before you can hit the target.

> Aiming lasers beyond 10K km probably requires gravitic lensing (hence
> we need gravity control).

Actually the limit is not *aiming* (though that's a major problem)
but beam focus. The size of the beam aperture sets a distance at
which the beam will start spreading badly no matter *what* you do.

Well, at least until you get beam densities so high the the photons
start to auto-collimate due to their mutual gravitational attraction.
But a beam that strong would have no trouble slicing a *planet* in
half.

> I'm assuming that railguns and the like over 500 or 1000 km probably
> require amazing predictive gunnery software to hit with because of
> travel time.

This is where we get into the fundamental principles of evasion in
space.

First you've got "tracking lag". Tracking is either using direct
emission from the ship, or reflected radar/lidar pulses. Both travel
at lightspeed.

So at 300,000 km, your tracking info is one second old. At 300 km.
it's one *millisecond* old. So that's "tracking lag".

In reality the lag will be longer due to things like signal
processing (ie how long does it take the sensor hardware & software
to turn the incoming signals to usable targetting info)

Second you've got "time of flight". That's how long it takes the
beam, missile or projectile to get from your ship to where you expect
the target to be.

Add the two together and you've got total lag. Which is also how much
time the target has to evade.

This is where a seemingly complex problem can be *drastically*
simplified. Rather than using *your* frame of reference, use the
frame of reference of the target.

Treat it as stationary in its frame of reference. In yours, it's
moving at a *constant* velocity on a particular heading. Yes, we
ignore acceleration for the moment.

By doing this, any evasion becomes acceleration from "at rest".
That's very easy to figure.

For now, we'll assume the ship can apply its full accel in any
direction (ie we ignore how long it may take to rotate the ship to
get the main drive pointing in the right direction)

So, for a laser, at 300,000 km, the ship has *two* seconds to evade
(one sec of tracking lag, plus 1 sec of flight time for the laser
pulses). We aren't counting detection time of the incoming pulses,
because the ship may be evading anyway, and they can detect the
pulses before they get there anyway. (for that matter, it's fairly
likely that they won't detect pulses that *miss* either)

so if the ship has 1 g of accel (we'll use 10 m/s^2 to make the math
simpler) in 2 seconds it can shift its center of mass by 20 meters
(d=0.5*a*t^2)

If the ship has a radius greater than that, you are pretty much
guaranteed a hit. If it's max dimension is onl 10 meters, there's a
fairly good chance you'll miss.

Particle beams travel a *lot* slower than light speed. So there's a
lot more "time of flight" lag. and for railguns, any feasible launch
velocity pretty much guarantees a miss at long ranges.

At shorter ranges the evasion time gets shorter. also note that
"jinking" as it is normally understoood is pretty useless. To
actually move your center of mass enough from your predicted location
requires *seconds* of thrust on a bearing. More seconds the bigger
the ship is (and it gets a lot worse when you include time to change
the ship's orientation)

For lasers, if the beam can stay focused that far out, you are in
trouble at a light second or more if your ship is at all large.

For particle beams the "oh shit" distance is a *lot* shorter.
Missiles, shorter still. And for railguns, you might as well not
bother unless you are stupid close.

Now flip this around and consider *defensive* fire.

Nothing you can do about lasers except use sand. Which quits covering
you if you change course or speed.

Ditto for particle beams.

Missiles you can try using your missiles, railguns and lasers on. And
the closer they get the less effectively they can dodge. Unless your
point defense is overwhelmed, or you are stupid close they aren't
very likely to get thru.

Railgun rounds are hard to spot (but give a very significant EM
signature when they are fired). Worse, lasers and particle beams may
not be very effective. You have to *destroy* the rounds. Even if you
melt them they are gonna hit and do damage.

Missiles are easier. If you knock out the electronics or drive/fuel
tanks, they can no longer manuever and can thus be ignored unless
they are still headed at you and are too close to dodge.

> Smaller ships jink easier. Saturation of cones from rapid fire guns
> may be one way to generate hits, but individual damage will be less
> than a single heavy penetrator but they will hit more readily.

At typical Traveller velocities, very few rounds are going to act as
"pentrators". At 3 km/sec relative velocity (5 minutes at 1 g) a
round will flash into plasma on impact releasing the enery of an
equal mass of TNT.

The impact energy goes up linearly with mass (ie twice as massive=,
twice the boom, 3 times as massive 3 times the boom etc)

But it goes up as the *square* of the relative velocity. so 2 twice
the velocity, 4 times the boom. Three times the velocity, 9 times the
boom.

since velocities of 300 km/sec are fairly normal, that is 100 times
the velocity or 10,000 times the boom. So a 50 gram .50 BMG bullet
wound release the energy of 5 metric tons of TNt on impact at that
velocity.

Not only would that make an impressive crater in the armor if it
didn't blast a hole in it, but it'd also cause massive spalling on
the inner surface.

If it does blow thru the armor, you'll have a multi thousand degree
jet of plama moving inwards at hundreds of km/sec.

As you can see the rules for hypervelocity projecftiles are *way*
different from anything we are used to currently.

> If you do have a heavily armoured large ship, you may be able to take
> some of the small stuff without too much damage. Fighters and ships
> may have (at least near term) delta-V constraints which will
> drastically limit battle plans, manouvering, and the ability of
> missiles, fighter, and other small craft (drones, etc) to speed
> towards an enemy that is manouvering and have enough fuel left to do
> final attack runs. Most battles will tend to look like jousts.
> Adjusting to change and pass again or pursue is a lot of delta V.

Keep in mind the sort of attack runs Babylon 5's fighters made. Since
orientaion has nother to do with velocity, when they could make
"close" passes on enemy ships, they'd just rotate the fighter to keep
the guns pointed at the enemy ship as they made a pass. Then it was
decelerate to a stop (far passt the target and accelerate back to the
battle for another pass.

Unless by some remarkable coincidence both sides have nearly the same
velocity (speed *and* direction) that's how most battles will go.
Fire at the enemy as you aproach, keep firing as you pass by (or
*thru* the enemy formation) and keep firing until you are out of
range.

Going back for another pass pretty much requires either mutual
consent *or* that one side has vastly superior acceleration.

And in case you missed it above, "sneak" attacks are moderately easy.
With
lasers you won't even know you are being fired upon unless there's
*very* near
miss or you sensors are looking *closely* at the firing ship at the
right time.

Particle beams are a lot "noiser" and likely to be detected unless
they miss by
a lot.

Missiles, probably fairly easy to spot.

Railgun or other dumb projectile weapon, you are unlikey to detect
the projectiles. Though there may be a firing signature (EM pulse in
the case of railgun, gas in case of conventional guns)

So if done right your first clue you are being attacked might be
lasers or projectiles blowing holes in the hull.
--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com